Solid-state radiation detector with improved sensitivity

ABSTRACT

A radiation detector includes a semiconductor substrate having opposing front and rear surfaces, a cathode electrode located on the front surface of the semiconductor substrate configured so as to receive radiation, and a plurality of anode electrodes formed on the rear surface of said semiconductor substrate. A work function of the cathode electrode material contacting the front surface of the semiconductor substrate is lower than a work function of the anode electrode material contacting the rear surface of the semiconductor substrate.

BACKGROUND OF THE INVENTION

The present invention generally relates to detectors for gamma-ray andX-ray detection devices.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to a radiation detectorcomprising a semiconductor substrate having opposing front and rearsurfaces, a cathode electrode located on the front surface of saidsemiconductor substrate configured so as to receive radiation, and aplurality of anode electrodes formed on the rear surface of saidsemiconductor substrate. The work function of the cathode electrodematerial contacting the front surface of the semiconductor substrate islower than the work function of the anode electrode material contactingthe rear surface of the semiconductor substrate.

Another embodiment of the present invention is a radiation detectorsystem comprising a semiconductor substrate having opposing front andrear surfaces, a cathode electrode located on the front surface of saidsemiconductor substrate configured so as to receive radiation, aplurality of anode electrodes formed on the rear surface of saidsemiconductor substrate, and a means for applying a forward bias to thedetector during operation such that the anode electrodes is maintainedat a higher potential than the cathode electrode and such that thesignal is collected from the anode electrodes. The work function of thecathode electrode material contacting the front surface of thesemiconductor substrate is lower than the work function of the anodeelectrode material contacting the rear surface of the semiconductorsubstrate.

Finally, another embodiment relates to a method of operating a radiationdetector comprising the steps of: a) providing a radiation detectorcomprising a semiconductor substrate having opposing front and rearsurfaces, a cathode electrode located on the front surface of saidsemiconductor substrate configured so as to receive radiation, and aplurality of anode electrodes formed on the rear surface of saidsemiconductor substrate; b) receiving radiation at the cathodeelectrode; c) applying a forward bias to detector to maintain the anodeelectrodes at a higher potential than the cathode electrode; and, d)collecting a signal from the anode electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a CZT substrate with anode electrodepixels.

FIGS. 2A-D are schematic cross-sectional views of a method of making adetector with a housing.

FIGS. 3A-I are schematic cross-sectional views of a method of making adetector at various stages in the formation of contacts thereon.

FIG. 4 is a graph showing I-V characteristics of a comparative devicehaving a cathode comprising gold, and not exposed to radiation.

FIG. 5 is a graph showing I-V characteristics of a comparative devicehaving a cathode comprising gold, and exposed to radiation.

FIG. 6 is a graph showing I-V characteristics of an embodiment of thepresent invention having a cathode comprising indium, and not exposed toradiation.

FIG. 7 is a is a graph showing I-V characteristics of an embodiment ofthe present invention having a cathode comprising indium, and exposed toradiation

FIG. 8 is a graph showing the interpixel resistance of edge pixels at ofan example embodiment of the present invention having differinginterpixel gap widths.

FIG. 9 is a graph showing the interpixel resistance of center pixels atof an example embodiment of the present invention having differinginterpixel gap widths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following definitions are used herein:

Cathode electrode: the electrode on one major surface of the detectorsubstrate where incident gamma rays or x-rays enter the detector, i.e.positioned towards the radiation source.

Anode electrodes: segmented electrode contacts located on the rearsurface of the substrate, i.e. positioned away from the radiationsource.

Forward-bias: The application of an electric potential between the anodeand cathode electrical contacts with the anode exposed to a higherpotential and cathode experiencing a lower potential. A forward-bias, asused herein, means that the anode is the electrode from which electricsignals, such as voltage or current generated by incident gamma rays orx-rays entering the detector, are collected.

Interpixel or inter pixel: the region or gap separating pixelelectrodes. For electrode configurations with non-pixellated discretecontact segments the term is equivalently applied to the gap betweencontact segments.

Solder mask: a coating on the semiconductor detector or on the printedcircuit board (“PCB”), which is designed to insulate and protect eitherthe segmented anode (pixels) on the semiconductor detector or the matingmetal pads on the PCB, keeping them from shorting during PCB-attachmentprocess. The solder mask may have any suitable color, such as a darkgreen/blue color and occasionally a yellowish color.

Embodiments of the present invention describe radiation detectors, suchas metal-semiconductor-metal (MSM) or heterojunction metal/semiconductortype detector with improved sensitivity. In other words, the detectorpreferably does not contain a p-i-n or p-n diode (i.e., no semiconductorp-n junction) formed in a semiconductor substrate or tile. In fact,radiation detectors of the embodiments of the present invention providethe additional benefit of room temperature operation without the need ofcooling as usually required for p-i-n heterojunction devices. Theembodiments of the invention provide the benefit of increased detectorsensitivity while also maintaining other beneficial properties such aslower leakage current and high detector energy resolution. Additionally,the embodiments of the invention may offer the further benefit of beingreliable and suitable for mass production. In one embodiment, aradiation detector comprises a semiconductor substrate having opposingfront and rear surfaces, a cathode electrode located on the frontsurface of said semiconductor substrate configured so as to receiveradiation, and a plurality of anode electrodes formed on the rearsurface of said semiconductor substrate. Preferably, the work functionof the cathode electrode material contacting the front surface of thesemiconductor substrate is lower than the work function of the anodeelectrode material contacting the rear surface of the semiconductorsubstrate. It is important to note herein that prior-art radiationdetector devices, such as that disclosed in for example T. Takahashi etal., “High-resolution Schottky CdTe diode for hard X-ray and gamma-rayastronomy”, Nuclear Instruments & Methods in Physics Research A 436(1999)111-119, have instead utilized indium as the anode electrodematerial rather than as the cathode electrode material, and applyforward and reverse bias so that the induced signal is collected fromthe anode and cathode, respectively. For example, a positive bias isplaced on the In anode in the device disclosed in the work of T.Takahashi et al. However, signal collection at the anode with indium asthe anode electrode material does not present the increased photo-peakcount sensitivity benefits of the configuration of the embodiments ofthe present invention.

Radiation detectors can be configured in a variety of ways. A commonconfiguration comprises a cathode electrode and a plurality of anodeelectrodes located on opposite sides of a semiconductor plate orsubstrate. Typically these radiation detectors have pixilated anodeelectrode arrays fabricated by various deposition and lithographyprocesses resulting in a gap between pixels, termed the interpixel gapor interpixel region. In an exemplary embodiment of the presentinvention, the interpixel gap has a width of between 100 and 500 μm.More preferably, the interpixel gap width is between 200 and 400 μm.

In the preferred embodiments, the radiation detectors comprise asemiconductor material, such as a semiconductor material preferablycomprising CdZnTe (CZT) or CdTe, having opposing front and rear surfacesand lacking a p-n or p-i-n junction. Although other types ofsemiconductor materials exemplified by lead iodide, thallium bromide,gallium arsenide or silicon may be used.

More preferred is Cd_((1-x)) Zn_(x) Te (where x is less than or equal to0.5), a wide band gap ternary II-VI compound semiconductor with uniqueelectronic properties. This type of semiconductor is useful in gamma-rayand X-ray detectors which are used as spectrometers that operate at roomtemperature for nuclear radiation detection, spectroscopy and medicalimaging applications.

Additionally, in the preferred embodiments, the radiation detectorscomprise a cathode electrode located on the front surface of saidsemiconductor substrate configured so as to receive radiation, and aplurality of anode electrodes formed on the rear surface of saidsemiconductor substrate. The work function of the cathode electrodematerial is preferably less than about 4.5 eV while the work function ofthe anode electrode material is preferably more than or equal to about4.8 eV. A cathode electrode material having a work function of less thanabout 4.5 eV may, for example, be selected from the group comprising oneof In, Al, and Ti and alloys thereof. An anode electrode material havinga work function of more than or equal to about 4.8 eV may, for example,be selected from the group comprising one of Au, Pt and other noblemetals and their alloys. In the embodiments of the present invention, byutilizing a cathode electrode material with a low work-function, such asa work function less than about 4.5 eV, for example, a cathode electrodematerial comprising indium, a higher density of charge injection, namelyelectron injection, occurs at the metal/semiconductor interface from thecathode.

In operation, the anode and cathode of the preferred embodiments iselectrically connected to a voltage source. The anode is exposed to ahigher potential than the cathode, for example, by applying a positivevoltage to the anode while the cathode is grounded or by applying anegative voltage to the cathode and virtually grounding the anode.

Illustrated in FIG. 1, is an example of pixellated anode electrodes 302formed on a semiconductor substrate 304, such as a CZT substrate (alsoreferred to as a “tile”). The cathode electrode is formed on the bottomside of the substrate 304.

FIG. 2A shows the radiation detector device of an embodiment of thepresent invention containing anode electrode pixels 400 on a rear sideof a CZT substrate 304, and containing cathode electrode 201 on a frontside of the CZT substrate. Optionally, a protective coating 420 isapplied to polished side edges of the CZT tile as shown in FIG. 2B. Forexample, the CZT tile may be dipped in a protective coating (such assolder mask) to cover the exposed sides and dried for at least 5 hours.

An optional housing is preferably formed separately and prior toattaching it to a radiation detector. The housing described in U.S. Pat.No. 7,462,833, which is hereby incorporated by reference, may be used asthe optional housing. Accordingly, the method of making the detector ofone embodiment comprises (a) providing a radiation detector comprising asemiconductor substrate having opposing front and rear surfaces, acathode electrode located on the front surface of said semiconductorsubstrate and a plurality of anode electrodes on the rear surface ofsaid semiconductor substrate, and wherein the work function of thecathode electrode material is lower than the work function of the anodeelectrode material, (b) providing a separately formed electricallyconductive housing, and (c) attaching the housing to the cathodeelectrode such that the housing and the cathode electrode are inelectrical contact. Alternatively, the optional housing may be omittedentirely or substituted with an optional printed circuit board type ofcathode plate, such as a gridded PCB type of cathode plate.

A non-limiting example of a method of particular embodiment is depictedin FIGS. 2A-2D showing side cross-sectional views of the detector atvarious stages of attaching a housing thereto. Starting with FIG. 2A, aradiation detector and its basic elements, cathode electrode 201,semiconductor substrate 304 and anode electrodes 400 are shown. Theanode and cathode electrodes may be formed in either order on substrate304 as will be described in more detail in FIG. 3. The detector may ormay not comprise at least one of a guard ring or screening electrode.Next, an optional protective coating (or solder mask) 420 is applied toedges of the substrate 304, as shown in FIG. 2B. Alternatively, thiscoating may be removed once a housing is formed thereon, resulting in anair gap between said housing and a side of the detector. Alternatively,this coating 420 is made of other electrically insulating materials,such as Humiseal.

As shown in FIG. 2C, the electrically conductive housing is attached tothe cathode and optionally the sides of the detector. In thisillustration, the housing 425 comprises a top portion 440 and anoptional side portion 430. The sides of the detector may or may not beglued to the protective coating (or solder mask) 420 covering sides ofthe detector depending on various reasons such as for example, if onewishes to later remove the protective coating. In this example, thehousing is attached to the cathode via an epoxy, although one skilled inthe art may chose from other adhesives.

The electrically conductive housing 425 shields the detector frombackground electromagnetic fields (or magnetic fields). Additionally,device electric fields are focused using this housing. The housing isalso preferably transparent to X-ray or gamma-ray radiation. Further,the housing preferably exhibits little or no oxidation in ambient air,such as under normal operating conditions of the detector. As such, thehousing is most preferably a thin structure and comprises a materialtransparent to radiation, substantially impervious to backgroundelectromagnetic fields and exhibits little or no oxidation at ambientconditions.

For example the housing may be between about 50 microns and 100 micronsthick. In some cases a metal foil is sufficient as a housing.

Based on the parameters set forth above, one skilled in the art maychose from a host of materials for constructing the housing. In general,metals and metallic alloys are preferred. Any suitable metal which doesnot substantially oxidize in air may be used. A non-limiting example ofsuitable metallic alloys includes stainless steel, brass (such Ni/Ticoated brass), NiCo alloys, NiFe alloys, NiFeCo alloys, NiFeMo alloys orNiFeCuMo alloys. A class of metal alloys termed “Mu-metals” is mostpreferred. Mu-metals are a type of NiFe alloy, particularly effective atscreening static or low frequency magnetic fields. In some cases, theaforementioned alloys may be doped with other alloying elements,mechanically pre-treated (e.g. cold worked, hot worked etc.), chemicallysurface-treated (e.g. surface coating for corrosion resistance) or anycombination thereof.

In a particular embodiment, the housing for a radiation detectorcomprises a first means for electrically contacting a cathode electrodeof a semiconductor radiation detector and a second means for shieldingat least one side of a detector. For example, the first means maycomprise the top portion 440 while the second means may comprise theside portion 430 of a housing 425. In some cases the side portion 430extends over a fraction of the thickness of the semiconductor substrate,on at least one side. However, the side portion 430 may be omittedentirely. The top portion 440 is preferably shaped to make optimalelectrical contact with a high voltage supply which is in electricalcontact with the anodes and cathodes of the radiation detector, andpreferably maintains the anode at a higher electrical potential relativeto the cathode.

In some cases a flat top portion 440 is preferred. In another particularembodiment, the housing 425 is hemispherical or dome-shaped andpartially or completely covers at least one side of the semiconductorsubstrate.

In some embodiments, the housing 425 is shaped to conform to geometry ofthe detector, more specifically, to geometry of the cathode, to which itis secured. Therefore, one skilled in the art may contemplate variouscurved or angular housing shapes given the shape of the detector. In anon-limiting example, the housing is a rectangular orcircular-cross-sectioned (e.g. cylinder) shape.

When the housing is constructed to extend over (partially or completely)at least one side of the substrate, said at least one side is spacedfrom said housing. This gap is either empty or filled with an insulatingmaterial.

The housing 425 is attached to the cathode electrode 201 such that anelectrical conduction path exists between the two. In a preferred case,the housing and the cathode are attached via an electrically conductivematerial. Most preferably, an electrically conductive polymericmaterial, such as a conductive epoxy applied to the inner face orsurface of the housing is used.

FIG. 2D illustrates an optional step where a solder mask 450 is disposedover the pixilated anode electrodes 400 providing mechanical protectionwhile allowing external access to the electrodes. Further electricalisolation of the anode electrodes may be achieved with the solder mask450 when the mask is formed between the pixilated anode electrodes 400at the interpixel region.

Accordingly, a particular optional embodiment is directed to a radiationdetector comprising a solder mask disposed above the anode electrodes inaddition to the housing 425 contacting the cathode electrode 201. Soldermasks suitable for embodiments of the present application are describedin U.S. application Ser. No. 11/642,819, filed on Dec. 21, 2006 which ishereby incorporated by reference in its entirely.

Preferably, the solder mask is photoimageable such that the portions ofthe solder mask 450 material over the anodes 400 are directly exposed toradiation, such as UV radiation, through a mask. The radiation eithercross links or uncross links the exposed portions of the solder mask,depending on the type of epoxy used (i.e., analogous to a positive ornegative photoresist). The uncross linked portions are then selectivelyetched away to form the openings 460 exposing a portion of the anode 400surfaces. Alternatively, for a non-photoimageable solder mask material,a conventional photoresist mask formed over the solder mask may be usedin the patterning step. The entire radiation detector device is coveredwith the solder mask except for the cathode and the anode regionsexposed through the opening. Preferably, only a portion of each anode400 is exposed in each opening 460 and no portion of the tile 304 isexposed. Thus, the solder mask is used as a protective coating (i.e.,passivation/encapsulation agent) for protecting the entire radiationdetector device.

As described in U.S. application Ser. No. 11/642,819, the radiationdetector device comprising a solder mask may be connected to a readoutprinted circuit board (PCB), at the underfill filling locations locatedon the mating pad. The solder balls are placed in the openings 460formed in the solder mask which serve as electrical interconnectsbetween anodes 400 of the detector device and the conductor pads of theprinted circuit board.

FIGS. 3A-I illustrate, without any intent to limit the presentembodiments, an example of steps in a method of forming tri-layer metalanode contacts on a semiconductor substrate at positions (pixels) fordefining radiation detector cells with an interpixel gap with highresistivity between the detector cells. While tri-layer contacts areillustrated, it should be understood that single layer or bi-layer anodeelectrodes may also be used. In this example, it is assumed that thesemiconductor substrate is made of cadmium zinc telluride (CdZnTe) orcadmium telluride (CdTe), although it will be appreciated that othersemiconductor materials, for example lead iodide, thallium bromide,gallium arsenide or silicon, can be used. Also, it will be assumed thatthe metal used for the metallization layer and the contacts is gold,although it will be appreciated that other metal anode, alloys or otherconductive materials, for example platinum, could be used instead.

Thus, FIGS. 3A-3I are a schematic cross-sectional views from the side ofa detector substrate at various stages in the formation of gold anodecontacts on a CdZnTe substrate. The detailed features and structure ateach step of the process are shown, resulting in an array of anodecontact pixels on the rear surface of the CZT (drawn as facing up inthis illustration), and a single cathode electrode on the front surfaceof the CZT tile (drawn as facing down in this illustration). In thisexample, two additional contact layers are added on to the pixilatedprimary contact layer on the rear side, for improved device assembly.The process can be applied to any array size and pixel configuration forCZT devices. A typical device size is a 20×20×5 mm detector, having 8×8pixels or 11×11 pixels depending on the application. As a precursor tocontact fabrication, the CZT wafer is polished and etched such that highquality clean crystal surfaces are prepared for the deposition process.

A direct lithography fabrication process is described with reference toFIGS. 3A-I, and for the case of the primary anode contact being gold,with two additional contact layers, and for separately forming of thecathode contact on the opposing side of the CZT tile or substrate 304shown in FIG. 3A. It is noted, however, that direct lithography processfor formation of the anode electrode is only one of many processes thatmay be used to form the electrodes. Other methods, including but notlimited to a lift-off method, may be used to form the electrodesinstead. For example, a lift-off method for forming the anode electrodesof the embodiments of the present invention comprises the steps of a)providing the substrate, b) forming a mask (such as a photoresist mask)over the substrate surface, where the pattern comprises portions of maskmaterial and spaces without mask material, c) depositing an anodeelectrode material over the mask where first portions of the anodeelectrode material are formed on the substrate in the spaces of the maskpattern while second portions of the anode electrode material are formedon the mask material, and d) removing or lifting-off the mask such thatsecond portions of the anode electrode material formed over the maskmaterial are also removed with the mask, and first portions of the anodeelectrode material contacting the substrate remain on the substrate.

In step 1 of the direct lithography process, shown in FIG. 3B a primarylayer of gold 200 is deposited onto a rear side of the CZT tile 304 (therear side is shown facing up in FIG. 3B). The gold layer 200 may bedeposited by electroless deposition. Alternatively the gold layer 200may be deposited by other known techniques, such as sputtering orevaporation. Additionally, a cathode electrode layer 201 comprisingindium is deposited by sputtering, electroless plating or other suitablemethod onto an opposing front side of the CZT tile. The CZT tiles arefirst cleaned in acetone, as is well known. The clean CZT tiles 304 areappropriately masked and placed in a chamber for sputtering or platingof gold onto a first side and indium onto a second side. Typicalthickness of the deposited cathode layer may be between 10-100 nm. Thedeposited gold may be annealed at 90 deg C. for 15 minutes to increaseadhesion to the substrate. An adhesion test can be done after a fewhours using Scotch tape to confirm quality of the adhesion.

In an optional step 2 shown in FIG. 3C, two additional contact layersare deposited onto the rear side (anode side to be pixilated) of thetile, over the primary contact layer 200. In this example, a Ni layer312 is deposited using sputtering or a thermal evaporation process to athickness <100 nm and nominally 50 nm. Then another gold layer 310 isdeposited using sputtering, thermal evaporation and/or an electrolessprocess to a thickness <50 nm and nominally 20 nm. Alternativeconductive contact material can be substituted for either or both of theadditional contact layers.

In step 3, as shown in FIG. 3D, a photoresist 202 is applied over thecontact layer(s). Tiles 304 are dipped in resist, for example Shipley1805 resist. Excessive resist is removed if necessary from the edgeusing a Q-tip, making sure the resist does not form any edge bead(especially on the pixilated face) as this would be detrimental for thepixel quality. Generally, the least possible amount of resist shouldremain on the pixilated face. The resist should be dried out for 10minutes with the pixilated face kept up and horizontal.

The resist coating is hardened in step 4 by baking for 10 minutes at 90°C. This step is done to drive excess solvent out of the resist. The tileis now prepared for lithography exposure.

In step 5, as shown in FIG. 3E a pixel pattern is formed on the rearside of the tile 304 by photolithography. A UV mask 204 is aligned overthe CZT tile surface, and the positive resist is exposed to UV. Thedirect lithography mask shades regions of the resist in a selected pixelpattern and exposes interpixel gaps to UV radiation. A contact mask isshown but other methods will work as well, such as proximity andprojection masks. A glass plate is placed on top making sure that theglass plate is horizontal. This ensures uniform contact between the tileand the mask. For the exemplary resist, exposure by a UV lamp (365 nmwavelength) for several minutes is suitable. If desired, a negativeresist may be used instead of the positive resist (in which case, theexposure mask's transparent and opaque regions are reversed).

In step 6 shown in FIG. 3F, the exposed photoresist is developed. Theresist developer (for example Microposit developer, MF-319) should coverthe tile(s). The tiles are placed into the developer with the pixilatedside facing up, developed for 2 minutes and the tile(s) are removed fromthe developer and rinsed in de-ionized water. The UV exposed resist isremoved, in preparation for creating the interpixel gap.

In step 7 the remaining resist pixel pattern 314 is baked for 20 minutesat 90° C. This step is done to harden the resist further.

In step 8, shown in FIG. 3G, the exposed contact regions 316 (notcovered by the pixel resist pattern 314) are etched. For the examplecontact materials, the following etching solution is suitable foretching through either just the primary contact layer or the optionalthree-layer contact. A 2% Br-Ethanol Glycol (BrEG) solution is preparedby pouring a 25 ml of Ethylene Glycol into a plastic beaker, then 0.5 mlof Bromine is added using a disposable pipette. Using the same pipette,the solution is mixed thoroughly until it becomes uniform. However, adifferent pipette or mixing device may also be used. Etching isconducted for approximately 3 minutes. This etching is done to removeunmasked interpixel contact material. To open the interpixel gap toachieve clean interpixel gaps, active spray agitation is performed.Disposable pipettes can be used to create Br-EG constant flow to agitatefor better etching. However, a different pipette or agitation or mixingdevice may also be used. The spray etching technique should rapidlyremove contact material flakes from the interpixel gaps, resulting inhigh interpixel resistance. The tiles are removed from the etchant andrinsed in deionized water.

In step 9 shown in FIG. 3H, the remaining resist is stripped using anacetone bath, resulting in tile 320 with a pixel array of contacts. Nophotoresist therefore remains on the CdTe or CdZnTe detector since it isusually a hygroscopic material that in time would absorb humidity anddeteriorate the detector performance.

The overall combination of depositing a metal layer having a workfunction greater than or equal to about 4.8 eV, such as gold, over therear substrate surface, depositing a metal layer having a work functionless than about 4.5 eV, such as indium, over the front surface, directphotolithography and the etching process results in the improved deviceperformance with minimal leakage current.

In optional step 10 shown in FIG. 3I, the primary contact material (inthis example gold) on the side of the tile 305 of the fabricated CZTdevice 322 is removed by side polishing. For example, the side of thetile(s) are first polished with 1200 grit then with 0.3 micron as finepolish. An alternate embodiment could, in step 1, mask the sides of theCZT tile instead of depositing gold on the sides. For this reason, theside contact removal step 10 may be optional. The resulting fabricatedCZT device has a cathode electrode 201 remaining on a front side, apixilated anode electrode array formed of a primary contact layer 200and secondary contact layers 312 and 310, separated by interpixel gaps316 on a rear side. FIG. 3I illustrates the multi-layer pixels as beingidentical width in cross-section for illustrative purpose.

EXAMPLES

Ohmicity is typically exhibited by a linear current-voltage (I-V)relationship. Indium as the cathode electrode material exhibits greaterohmicity as compared to gold. As shown in FIGS. 4-7, current wasmeasured at the cathode between 0-5 V for comparative devices andexamplary embodiments of the present invention with and without beingirradiation by a Co-57 source. A linear regression analysis wasperformed with R² which is used to determine the linearity of the curve.It is noted that a maximum of R²=1 represents a perfectly linear curveand therefore, perfect ohmic character.

As shown in FIG. 4, a regression analysis indicates that a comparativedevice comprising gold as the cathode electrode material and not exposedto radiation from a Co-57 source results in an R² value equal to 0.8532.As shown in FIG. 5, when the device of FIG. 4 is irradiated, aregression analysis of the I-V relationship shows a resulting R² valueequal to 0.7861 which indicates that ohmicity of the gold cathode hasdecreased.

As shown in FIG. 6, a regression analysis shows that an exemplary devicecomprising indium as the cathode electrode material and not irradiatedby a Co-57 source results in an R² value equal to 0.9996 indicative of ahigh degree of linearity for the I-V relationship. As shown in FIG. 7,when the device of FIG. 6 is exposed to radiation, a regression analysisindicates that ohmicity of the indium cathode decreases only veryslightly resulting R² value equal to 0.9991, still indicative of a highdegree of linearity and therefore, Ohmic character.

It is clear from FIGS. 4-7 that by replacing gold as the cathodeelectrode material with indium, the embodiments of the present inventionoffer the benefits of enhanced charge collection when operated as aradiation detector for example.

In order to improve the photo-peak counts, the inventors have discoveredvarious configurations to achieve improved sensitivity of the detectors.For example by changing the material of one of the electrodes, namely bychanging the cathode electrode material from gold to indium, the resultis increased electrode ohmicity. Also, by changing the active volume ofthe detector, for example by decreasing the interpixel gap width, animproved device performance results.

To achieve these results, radiation detectors comprising a semiconductorsubstrate having opposing front and rear surfaces, a cathode electrodelocated on the front surface of said semiconductor substrate configuredso as to receive radiation, and a plurality of anode electrodes formedon the rear surface of said semiconductor substrate were manufactured.Various elements of the device were changed for a particularexperimental run. For example, the interpixel gap was changed frombetween 600 μm to 460 μm, maintaining the pitch at 2.46 mm, on adetector with gold cathode and anode electrodes. Subsequently, keepingthe pixelation on the anode untouched, the cathode was polished off, andmetalized with indium. As the last step, the cathode was protected, andthe anode was re-fabricated with 300 μm interpixel gap.

The following are non-limiting examples detailing particular deviceconfigurations of the present radiation detector devices of theembodiments of the invention:

Comparative Example 1 In Cathode and Au Anode with 460 μm Interpixel Gap

As a first comparative example of a radiation detector device, a 20×20×5mm³ radiation detector device with an 8×8 pixel configuration is listedin Table 1 as CE 1. The device comprises: a semiconductor substratehaving opposing front and rear surfaces, a cathode electrode comprisinggold located on the front surface of said semiconductor substrateconfigured so as to receive radiation, and a plurality of anodeelectrodes comprising gold formed on the rear surface of saidsemiconductor substrate. The plurality of anode electrodes are formed aspixels having an interpixel gap width between each pixel of about 600μm. The anode and cathode electrodes are electrically connected to avoltage source which provides a means for applying forward-bias electricpotential of about 400 V while a Co-57 source is placed 25 mm from thecathode.

Comparative Example 2 In Cathode and Au Anode with 300 μm Interpixel Gap

As a second comparative example of a radiation detector device, a20×20×5 mm³ radiation detector device with an 8×8 pixel configuration islisted in Table 1 as CE. The device comprises: a semiconductor substratehaving opposing front and rear surfaces, a cathode electrode comprisinggold located on the front surface of said semiconductor substrateconfigured so as to receive radiation, and a plurality of anodeelectrodes comprising gold formed on the rear surface of saidsemiconductor substrate. The plurality of anode electrodes are formed aspixels having an interpixel gap width between each pixel of about 460μm. The anode and cathode electrodes are electrically connected to avoltage source which provides a means for applying forward-bias electricpotential of about 400 V while a Co-57 source is placed 25 mm from thecathode.

Example 1 In Cathode and Au Anode with 460 μm Interpixel Gap

As a first example of a non-limiting embodiment of the presentinvention, a 20×20×5 mm³ radiation detector device with an 8×8 pixelconfiguration is listed in Table 1. The device of Ex 1 comprises: asemiconductor substrate having opposing front and rear surfaces, acathode electrode comprising indium located on the front surface of saidsemiconductor substrate configured so as to receive radiation, and aplurality of anode electrodes comprising gold formed on the rear surfaceof said semiconductor substrate. The plurality of anode electrodes areformed as pixels having an interpixel gap width between each pixel ofabout 460 μm. The anode and cathode electrodes are electricallyconnected to a voltage source which provides a means for applyingforward-bias electric potential of about 400 V while a Co-57 source isplaced 25 mm from the cathode.

Example 2 In Cathode and Au Anode with 300 μm Interpixel Gap

As a second example of another non-limiting embodiment of the presentinvention, a 20×20×5 mm³ radiation detector device with an 8×8 pixelconfiguration is listed in Table 1. The device of Ex 2 comprises: asemiconductor substrate having opposing front and rear surfaces, acathode electrode comprising indium located on the front surface of saidsemiconductor substrate configured so as to receive radiation, and aplurality of anode electrodes comprising gold formed on the rear surfaceof said semiconductor substrate. The plurality of anode electrodes areformed as pixels having an interpixel gap width between each pixel ofabout 300 μm. The anode and cathode electrodes are electricallyconnected to a voltage source which provides a means for applyingforward-bias electric potential of about 400 V while a Co-57 source isplaced 25 mm from the cathode. It is noted that the same Co-57 sourcewas used for all measurements, the measurements were made within a fewweeks of each other, and the measurements were taken based on a sourcestrength of about 17 μCi.

TABLE 1 Cathode/ Average Interpixel Anode photo-peak % Increase gapElectrode Bias counts of over adjacent Example (μm) Material (Volts)center pixels configuration CE 1 600 Au/Au 400 2437 — CE 2 460 Au/Au 4002726 12 Ex 1 460 In/Au 400 3023 11 Ex 2 300 In/Au 400 3317 10

Between the configurations of CE 1 and CE 2 as shown in Table 1, anaverage increase in photo-peak counts of 12% is observed.

Between the configurations of CE 2 and Ex 1 as shown in Table 1, anaverage increase in photo-peak counts of 11% is observed.

Between the configurations of Ex 1 and Ex 2 as shown in Table 1, anaverage increase in photo-peak counts of 10% is observed.

The results shown in Table 1 may be summed up as an effective increasein photo-peak counts of 30-50% in advancing the configuration of CE 1 tothat of Ex 2.

Table 2 shows the change in the photo-peak counts of the center pixelsbetween the configurations of CE 1 and CE 2 for various radiation sourcestrengths. In the experiments yielding the results shown in Table 2,only the center pixels are considered to eliminate the edge effect fromthe analysis as the edge pixels did not show the expected change whenthe radiation source strength was doubled. As shown in Table 2, anincrease in the photo-peak counts ranging from 2-23% results when theinterpixel gap is changed from a width of 600 μm to 460 μm. Thisindicates that reducing the interpixel gap, while maintaining the otheraspects of configuration, such as pitch, is advantageous to enhance thesensitivity of the detector.

TABLE 2 Example Configuration Photo-peak coun

% Change CE 1 Au—Au 600 μm 2650 — CE 2 Au—Au 460 μm 2690  2 CE 1 Au—Au600 μm 1658 — CE 2 Au—Au 460 μm 2000 21 CE 1 Au—Au 600 μm 2913 — CE 2Au—Au 460 μm 3113  7 CE 1 Au—Au 600 μm 2529 — CE 2 Au—Au 460 μm 3102 23

indicates data missing or illegible when filed

In the next stage, the cathode electrode of the four 460 μm interpixelgap width detectors was changed to In. This was done by maintaining thefabrication by protecting the anode, and suspending the cathode in anelectroless In solution. Table 3 shows the results of this change in theconfiguration.

TABLE 3 Example Configuration Photo-peak coun

% Change CE 2 Au—Au 460 μm 2690 — Ex 1 In—Au 460 μm 3230 20 CE 1 Au—Au460 μm 2000 — Ex 1 In—Au 460 μm 2827 41 CE 1 Au—Au 460 μm 3113 — Ex 1In—Au 460 μm 3392  9 CE 1 Au—Au 460 μm 3102 — Ex 1 In—Au 460 μm 2644−14  

indicates data missing or illegible when filed

In the final stage, the anode electrode of the four detectors wasre-fabricated with a smaller interpixel gap of 300 μm. Table 4 shows theresults of this step. An increase in photo-peak counts of 7-17% is seenwith this change. This indicates that the 300 μm interpixel gap is thebest for improvement of sensitivity for this particular detectorconfiguration. In general, the gap width may range from 100-500 microns,such as 200-400 microns, depending on other detector materials anddimensions.

TABLE 4 Example Configuration Photo-peak coun

% Change Ex 1 In—Au 460 μm 3230 — Ex 2 In—Au 300 μm 3522 9 Ex 1 In—Au460 μm 2827 — Ex 2 In—Au 300 μm 3032 7 Ex 1 In—Au 460 μm 3392 — Ex 2In—Au 300 μm 3622 7 Ex 1 In—Au 460 μm 2644 — Ex 2 In—Au 300 μm 3094 17 

indicates data missing or illegible when filed

It is noted that for additional examples of the embodiments of theinvention disclosed herein, the interpixel gap was reduced further to100 μm but the measured interpixel resistance of these detectors, onaverage, were 2 orders of magnitude smaller than for embodiments of thepresent invention comprising a 460 μm interpixel gap width. In addition,the resistance profile showed a sharp decrease at higher biasesindicating the possibility of electrical breakdown. For example, FIG. 8shows a comparison of the interpixel resistance of edge pixels of theexample embodiments of the present invention comprising 460 μm, 300 μmand 100 μm interpixel gap widths. FIG. 9 shows a similar comparison ofthe center pixels of detectors in these configurations. As noted above,the center pixels typically have the best interpixel resistance in theentire detector. Moreover, while the examplary embodiment comprising a300 μm interpixel gap widths exhibit marginally lower resistance thanthe examplary embodiment comprising a 460 μm interpixel gap width, theresistance profile is still sufficiently flat to ensure electricalstability. Conversely, the examplary embodiment comprising a 100 μminterpixel gap width exhibits a resistance two orders of magnitude lowerthan that of the examplary embodiment comprising the 460 μm interpixelgap width, which tends to decrease very sharply towards higher biases.In other words, while the examplary embodiment comprising a 100 μminterpixel gap width shows superior sensitivity, it has poor interpixelresistance, and the interpixel resistance is a very important factor inthe energy resolution performance of radiation detectors.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the present invention is not so limited. It willoccur to those of ordinary skill in the art that various modificationsmay be made to the disclosed embodiments and that such modifications areintended to be within the scope of the present invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

1. A radiation detector, comprising: a semiconductor substrate havingopposing front and rear surfaces; a cathode electrode located on thefront surface of said semiconductor substrate configured so as toreceive radiation; and a plurality of anode electrodes formed on therear surface of said semiconductor substrate; wherein a work function ofthe cathode electrode material contacting the front surface of thesemiconductor substrate is lower than a work function of the anodeelectrode material contacting the rear surface of the semiconductorsubstrate.
 2. The radiation detector of claim 1, wherein the workfunction of the cathode electrode material is less than about 4.5 eV,and the work function of the anode electrode material is greater than orequal to about 4.8 eV.
 3. The radiation detector of claim 1, wherein thecathode electrode material comprises one of In, Al, and Ti.
 4. Theradiation detector of claim 1, wherein the anode electrode materialcomprises one of Au and Pt.
 5. The radiation detector of claim 1,wherein the semiconductor substrate comprises CdTe or CZT, and thesubstrate lacks a p-n or p-i-n junction.
 6. The radiation detector ofclaim 1, wherein the plurality of anode electrodes are configured aspixels with an interpixel gap between each two adjacent anode electrodepixels.
 7. The radiation detector of claim 6, wherein said interpixelgap width is between 100 and 500 μm.
 8. The radiation detector of claim7, wherein said interpixel gap width is between 200 and 400 μm.
 9. Theradiation detector of claim 1, further comprising an electricallyconductive housing located in electrical contact with the cathodeelectrode.
 10. The radiation detector of claim 1, further comprising avoltage source electrically connected to the anode and the cathodeelectrodes.
 11. A radiation detector system, comprising: a semiconductorsubstrate having opposing front and rear surfaces; a cathode electrodelocated on the front surface of said semiconductor substrate configuredso as to receive radiation; a plurality of anode electrodes formed onthe rear surface of said semiconductor substrate, wherein a workfunction of the cathode electrode material contacting the front surfaceof the semiconductor substrate is lower than a work function of theanode electrode material contacting the rear surface of thesemiconductor substrate; and a means for applying a forward bias to thedetector during operation such that the anode electrodes is maintainedat a higher potential than the cathode electrode and such that thesignal is collected from the anode electrodes.
 12. The radiationdetector system of claim 11, wherein the work function of the cathodeelectrode material is less than about 4.5 eV, and the work function ofthe anode electrode material is greater than or equal to about 4.8 eV.13. The radiation detector system of claim 11, wherein the cathodeelectrode material comprises one of In, Al, and Ti, the anode electrodematerial comprises one of Au and Pt, and the semiconductor substratecomprises CdTe or CZT.
 14. A method of operating a radiation detector,comprising: providing a radiation detector comprising: a semiconductorsubstrate having opposing front and rear surfaces; a cathode electrodelocated on the front surface of said semiconductor substrate configuredso as to receive radiation; and a plurality of anode electrodes formedon the rear surface of said semiconductor substrate, wherein a workfunction of the cathode electrode material contacting the front surfaceof the semiconductor substrate is lower than a work function of theanode electrode material contacting the rear surface of thesemiconductor substrate; receiving radiation at the cathode electrode;applying a forward bias to detector to maintain the anode electrodes ata higher potential than the cathode electrode; and collecting a signalfrom the anode electrodes.
 15. The method of claim 14, wherein the workfunction of the cathode electrode material is less than about 4.5 eV,and the work function of the anode electrode material is greater than orequal to about 4.8 eV.
 16. The method of claim 14, wherein the cathodeelectrode material comprises one of In, Al, and Ti, the anode electrodematerial comprises one of Au and Pt, and the semiconductor substratecomprises CdTe or CZT.
 17. The method of claim 14, wherein said forwardbias injects electrons from said cathode electrode material into thefront surface of the semiconductor substrate.
 18. The method of claim14, wherein the radiation comprises at least one of gamma ray and X-rayradiation.
 19. The method of claim 14, wherein the plurality of anodeelectrodes are configured as pixels with an interpixel gap between eachtwo adjacent anode electrode pixels and the signal comprises a measuredcurrent or voltage which corresponds to the radiation received at eachpixel.
 20. The method of claim 19, wherein said interpixel gap width isbetween 100 and 500 μm.